1. Field of the Invention
The present invention relates to an improvement in a closed mixing machine, and more particularly a mixing machine for mixing a rubber material or a plastic material.
2. Description of the Prior Art
A closed mixing machine is adapted to perform a batch mixing of rubber or a plastic material. More particularly, the machine allows for a plasticization mixing of the rubber material, and batch mixing of a carbon master with or without a sulfurizing agent in manufacturing rubber products such as a tire.
There are a number of conditions required in such mixing machine, particularly a reduced period of mixing time for improving productivity, an increased degree of dispersion of additives for achieving improved mixing, and prevention of excessive heat generation of the mixing material.
The mixing performed by the batch-type mixing machine includes basic processes consisting of macro-dispersion, distributive mixing, and micro-dispersion of the material. Macro-dispersion is mainly caused by an axial propulsive force created by helical vanes of a rotor rotatable in a mixing chamber, while the micro-dispersion thereof is caused by a strong shearing force of the vanes acting on the material moving in a direction normal to the rotor axis, as disclosed in Japanese Patent Laid-Open Publication Nos. 58-4567, 58-887, 58-888, 58-5094 and 59-31369.
The conventional mixing machines proposed in the foregoing documents provide improvements relating only to the macro-dispersion by modifying a length, helical angle of the rotor vanes, and a ratio of the length to the diameter of the mixing chamber. Such improvements were sought in view of the necessity of higher productivity and improved mixing quality, and for keeping the material at a relatively low temperature during the mixing operation. However, the foregoing mixing machine fails to provide a suitable arrangement for performing a desired effective micro-dispersion of the material.
With reference to FIGS. 1 to 4 of the accompanying drawings, the micro-difusion is described more specifically hereinbelow.
FIG. 1 shows the manner in which the material flows in the direction normal to the rotor axis of the conventional mixing machine and also the manner in which shearing stress works on the mixing material. The shearing stress .tau. varies in strength at different points distributed on a plane normal to the axis of the rotor 1. A greater shearing force or stress created in a region near a rotor tip serves to shear and plasticize the mixing material such as a rubber material and to disperse additives such as carbon black. In FIG. 1, reference numerals 2, v, ho, h and .theta. indicate a wall of the mixing chamber, rotor speed, a tip clearance, a distance between the rotor front face and the chamber wall, and a front (inclusion) angle, respectively.
It is important to disperse the additives sufficiently in the mixing material in manufacturing tires. In order to achieve sufficient dispersion of the additives, the rotor needs to apply a shearing force greater than a determined minimum shearing stress .tau.c to the material. Accordingly, an increased shearing stress is required to achieve an improved micro-dispersion.
In the case of mixing of the rubber material, the shearing stress .tau. is given by the following expression (1) and if a drag flow concept can be adopted, a rate of shear .gamma. is given by the following expression (2): ##EQU1## where n denotes a viscosity index greater than 0, R is a rotor diameter, N is a rotor rotation speed (rpm), and K is a viscosity coefficient (which decreases as a temperature of the rubber material increases). In view of the foregoing behavior of the mixing material, the following arrangement can be envisaged to improve the micro-dispersion of the mixing machine:
(A) Increasing the shearing stress .tau. in an overall cross section normal to the rotor axis while enlarging a region permitting .tau.&gt;.tau.c.
(B) Increasing an opportunity for the material to pass through the region permitting .tau.&gt;.tau.c near the rotor tip.
The arrangement (A) allows for increasing .tau. of the expression (1) as a whole of a shearing region, and more particularly, from the arrangement (A) the following arrangement can be envisaged:
(A-1) Increasing N of the expression (2) with a determined cross-sectional shape of the rotor unchanged.
(A-2) Decreasing ho of the expression (2) with a determined N.
(A-3) Decreasing h and hence .theta. with a determined N and ho.
These arrangements have been practiced independently or jointly in the conventional mixing machine, however, they did not exhibit an advantageous result for the reasons described hereinbelow.
The arrangements (A-1) and (A-2) have a drawback in that a maximum shearing stress .tau.max which can be obtained at the tip region increases to such an extent that an excessive torque is caused at the time of charging the material in the initial mixing operation, and a rapid increase of heat occurs to thereby decrease .tau. soon due to the fact that the viscosity coefficient K decreases as the temperature of rubber material increases. As a result, the micro-dispersion cannot be improved. Such a drawback becomes serious if the material is required to be mixed at a lower temperature.
The arrangement (A-3) has a drawback in that the rotor fails to pull the material into the chamber at the initial stage of mixing operation, and requires an increased period of time for completing the operation.
As understood from the foregoing description, application of the arrangement (A) for increasing N and decreasing h is restricted to a certain extent. In view of such restriction, the batch-typed mixing machine for mixing rubber materials needs to be designed and manufactured based on empirically established standards.
FIG. 2 shows a part of the specifications for designing the conventional batch-typed mixing machine, in which the tip clearance ho and the rotation speed N are determined on the basis of a certain standard regardless of the rotor diameter R, and specifically N is 40-70 rpm.
This arrangement is apparent from FIG. 3 which shows that the maximum shearing speed .tau.max (only at the rotor tip region) is lower than 350 sec.sup.-1, and the ratio .alpha. of the tip clearance ho to the rotor diameter R, i.e. ho/R=.alpha., is set to be 0.01-0.015 regardless of the size of the mixing machine.
The arrangement (B) seeks to increase the amount of the material which undergoes shearing stress .tau. greater than the determined shearing stress .tau.c (.tau.&gt;.tau.c) without increasing .tau. as exerted by the arrangement (A). Specifically, the following arrangement is envisaged:
(B-1) Increasing a number of rotor vanes to thereby allow an increased amount of the material to be subject to the shearing stress.
According to the arrangement (B-1), a pair of twin-vane rotors 1 each having a shorter vane 3 and a longer vane 2 and being rotatable in a mutually opposite direction as shown in FIG. 4a are replaced by a pair of quadruple-vane rotors 1 each having two shorter vanes 3 and two longer vanes 2, each rotor being rotatable in a mutually opposite direction. The quadruple-vane rotors complete the mixing for a period of time shorter than the twin-vane rotors, and increases the productivity by 20% in the break-down mixing of a natural rubber and in the master batch mixing of a carbon black.
The provision for the increased number of vanes, however, makes an effective mixing space of the mixing chamber smaller, and thus decreases an axial flow rate of the material to thereby impair the degree of mixing. As a result, the shape of the vanes needs to be modified.
To provide a further increased number of rotor vanes, for instance, to form a six-vane rotor or a quadruple-vane with additional vanes, would increase the possibility of occurrence of the foregoing drawback.
The above-described drawbacks in the conventional arrangements to improve the micro-dispersion of the material are summarized as follows:
The greater shearing stress by decreasing ho and by increasing N causes an excessive torque and excessive heat generation. The increased number of rotor vanes increase the opportunity for the material to undergo shearing stress and impairs the degree of mixing.